The objective is a structure determination of a cytolytic toxin from the sea anemone Stoichactis Helianthus. Crystals of the 16,000 Dalton protein, complexed with a detergent (Beta-octyl-glucopyranoside), diffract X-rays to 1.5 angstrom resolution. A properly refined model of the cytotoxin will have atomic positions determined to within an uncertainty of 0.1 angstrom and will deviate from ideal geometry by no more than 0.015 angstrom and 3 degrees in bond length and angle, respectively. A cytotoxin structure, (1) will contribute information of high accuracy and great detail to the data base for protein structure, (2) will assist interpretation of results of experiments aimed at the identification of specific regions of the cytotoxin which are responsible for its properties of cytolysis and aggregation and, (3) will provide a wealth of information as to the interaction of detergent with hydrophobic surfaces of proteins. The three dimensional structure should provide a basis for postulating reasonable mechanisms for the entry of proteins into membranes as occurs during viral infection of cells. Finally, as more structures of membrane proteins become known, the cytotoxin may contribute to the discovery of common structural features of proteins which bind to or reside in membranes. Well established methods of X-ray diffraction will be used to determine the cytotoxin structure. Heavy atom derivatives will be necessary for phase information. The initial phase estimates will be improved by least squares refinement of a model against the full extent of the diffraction data.